Spotting

I am currently 13 weeks with twins. At ten weeks, I woke up one morning to very heavy bleeding. I went to the ER and the babies were perfectly fine. My doctor did an exam right after I left the ER and said it could happen again, but if there is no pain do not worry about it. The next day I had bleeding again, but not as heavy, and then again a few days later. Ever since I have had brown spotting. I had a doctors appointment this past Friday and she said everything looks good and not to worry about the spotting. Just wondering if anyone else has experienced anything like this. It's hard not to worry when they don't have any explanation for it and it's been going on for three weeks now.